Recently, in the mobile communication field, attentions are drawn to the radio communication technology operable on the impulse in the UWB or the like, as a communication scheme realizing high speed and low power consumption.
FIG. 1A and 1B are block diagrams showing an arrangement of a UWB communication apparatus using the conventional biphase modulation. (See Intel Technology Journal Q2, 2001 “Ultra-wideband Technology for Short- or Medium-Range Wireless Communications”.
In the transmitter (FIG. 1A), impulse generating means 101 generates a brief single pulse at a constant interval. The mixer 103 changes the impulse polarity in accordance with the polarity of transmission data 102, thereby effecting biphase modulation. The biphase modulation wave is made into a desired band signal by an impulse shaping filter 104 and then sent out at a transmission antenna 105.
At the receiver (FIG. 1B), the signal received at the reception antenna 106 is amplified up to a desired intensity by an LNA 107. The mixer 109 mixes the reception signal with the version of reception signal passed through delay means 108 for causing a signal delay at the pulse interval given by the transmission means, to effect differential detection thereby detecting a code change between adjacent pulses. The detection result is digital-processed in a data demodulating means 110, to reproduce transmission data.
The UWB scheme thus configured has the following merits.
(1) Low Power Consumption
Because of the scheme not using the carrier wave always requiring continuous output, less power is needed in transmission. This enables to reduce apparatus consumption power.
(2) Small Size and Low Price
Because of no need of analog RF components difficult in CMOS fabrication into an IC, such as VCOs and FR filters, and of not a circuit configuration requiring linearity, CMOS IC fabrication is suitably applicable to facilitate apparatus size and price reduction.
(3) Because of the broad frequency band owing to the communications using high-speed impulses, high-speed data communications are available. In UWB communications using microwave band (3–10 GHz), high-speed data communications are feasible at approximately 100 Mbps.
The circuit schemes shown in FIG. 1A, 1B are suited for IC fabrication. Nevertheless, the impulse shaping filter 104 for restricting the transmission band is an RF-frequency band filter. Usually, it often uses an RF element such as SAW, hence being difficult in IC fabrication.
Accordingly, in order to IC-fabricate the circuit entirety, there is a need to eliminate the impulse shaping filter 104 from the circuit construction. For doing so, it is satisfactory to form a band-limited waveform within the impulse generating means 101.
Generally, in an arbitrary band frequency, the impulse waveform F(t) having an in-band center frequency F0 and band width W is defined by Equation 1.F(t)=[sin(2πWt)/(πt)]cos(2πF0t)  Equation 1
Accordingly, for example, with an arrangement that a D/A converter 112 is caused to generate a voltage value in accordance with a waveform table stored in a memory 113 in the timing of a rectangular wave of from a rectangular wave generator 111 thereby generating an impulse waveform F(t) by the D/A converter 112 as shown in FIG. 2, it is possible to configure a circuit not requiring an impulse shaping filter but suited for IC fabrication.
However, this arrangement requires a sampling rate several times the band frequency of the impulse waveform generated by the D/A converter 112. For example, in order to generate an impulse waveform in a band of 3 to 10 GHz, there is a need of a sampling rate at several tens GHz.
In the nowadays device technology, there are no D/A converters operating at such high frequency. Should available in the future, it is considered difficult to reduce consumption current because of high switching frequency. Namely, there is a setback that it is difficult to realize the consumption power reduction, the greatest merit in the UWB.